Getting the 'blink' out: Steady nanoparticle emitters emerge

PORTLAND, Ore.  Nanoparticles offer advantages over traditional semiconductor LEDs and lasers in terms of size, ability to change color by merely changing their size and lower fabrication costs. But those advantages were often offset by a tendency toward random blinking.

Now, researchers claim to have demonstrated always-on nanoparticle emitters in a collaborative effort among the University of Rochester, Cornell University, Eastman Kodak Co. and the Naval Research Laboratory. By fabricating a new type of nanoparticle with a smooth gradient between two different materials, the researchers said the nanoparticles emit steadily like traditional LEDs and lasers.

"We were making nanoparticles with two different materials when we noticed that one formulation was not blinking," said University of Rochester professor Todd Krauss. "What we think we have here is a smooth transition between the two materials rather than the usual sharp interface between the inner and outer shells."

New nanoparticles use a smooth transition between different semiconducting materials to prevent blinking (Image by Ted Palwicki, University of Rochester)

Nanoparticles are often constructed from concentric shells to customize their light-emitting characteristics and to isolate a toxic inner material or to prepare the outer surface for a particular application. The location where two materials touch is usually the same sort of abrupt interface as between two semiconducting materials on a chip. However, these abrupt interfaces are now thought to be the cause of blinking.

The non-blinking nanoparticles discovered by the U.S. researchers have a smooth gradient between materials, achieved by annealing.

"We were experimenting with different methods, and we heated these particular nanoparticles to see if that would improve their performance," said Krauss. "Now we think the heating caused the two materials to diffuse into one another, creating a smooth transition between them."

The formulation currently being used has an inner core of cadmium selenium and an outer shell of zinc selenium. When energy was added to the nanoparticles, their electrons jump to a higher orbit, then fall back emitting a photon to shed the excess energy, constantly emitting light with no blinking.

When a sharp interface was used between the materials, their efficiency dropped. The reason is that at random intervals the excited electrons shed their energy by vibrating the atomic lattice, thereby creating heat. When the particles were annealed, the two materials diffused into one another, creating a smooth transition that raised their efficiency by favoring light emission to shed the excess energy rather than non-radiative heat emission.

The researchers are currently experimenting with other materials to determine whether they can duplicate the non-blinking behavior by creating smooth gradients.

Funding was provided by the Eastman Kodak, the U.S. Energy Department, the National Science Foundation, the University of Rochester's Center for Electronic Imaging Systems, the Cornell University Center for Nanoscale Systems, the Office of Naval Research and the Alexander von Humboldt Foundation.